1. Field of the Invention
The present invention relates to a communication system, a child station and a control method thereof, and a storage medium.
2. Description of the Related Art
A variety of wireless communication protocols are defined with regard to a wireless communication system that performs a Time Division Multiple Access connection communication that forms a wireless network; for example, IEEE Std. 802.15.3-2003, or Bluetooth Specification Version 1.0B. When transitioning to a power save mode, a child station informs a control station that the child station itself is in the power save mode, and the control station, upon receiving a request from the child station, changes access timing from the child station.
For example, there are four power management modes under the IEEE Std. 802.15.3, as follows:    Active;    Asynchronous Power Save (APS);    Device Synchronous Power Save (DSPS); and    Piconet Synchronous Power Save (PSPS).
The three modes other than “ACTIVE” are energy conservation modes.
Under the APS mode of the IEEE 802.15.3 specification, a DEV, that is, a child station terminal device, maintains a communications state by accessing a PNC, that is, a control station terminal device, at an arbitrary time within an ATP interval. ATP stands for “Association Timeout Period,” which is a time period, that is, a time-out value, wherein an association, that is, a connection state, is sustained when a signal is not received from a communication participant. When the signal is not received from the communication participant, the association is maintained until a time interval wherein the signal is not received exceeds the ATP, whereupon the association is terminated. A frame for maintaining the communication state between the DEV and the PNC under the present specification is a Contention Access Period (CAP) or a Channel Time Allocation (CTA) that is addressed to the channel.
The CAP uses a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. Accordingly, when the DEV fails to access the PNC because it conflicts with another DEV in attempting to access the PNC, it is necessary for the DEV to retry until it succeeds in access the PNC.
When using the CAP, it is necessary for the DEV to analyze a beacon frame and acquire access timing for the CTA that is addressed to the channel. Consequently, when the CTA that is addressed to the channel is not present within the frame thus received, the DEV must persist in the operation of receiving and analyzing the beacon frame until the DEV finds the CTA that is addressed to the channel.
In such a circumstance, a problem arises wherein a power consumption of the DEV increases significantly. Japanese Patent Laid Open No. 2003-517741 discloses a system that specifies a frequency whereby a mobile terminal device that suspends an operation and transitions to the power save mode when a period that a base station specifies ends listens for a call message from the base station.
The configuration recited by Japanese Patent Laid Open No. 2003-517741 presumes the frequency of listening for the call message from the base station to be an integer multiple of a superframe. A specification such as the IEEE 802.11 wireless LAN (Wi-Fi) standard does not set forth a time-out of the Association Timeout Period, and thus, it is possible to set the frequency not to time out. With a system such as the IEEE Std. 802.15.3, however, which does have a time-out for the Association Timeout Period between the PNC and the DEV, it is not possible to set an arbitrary value of the frequency of listening for the call message from the base station.
Consequently, with a conventional configuration the DEV is not capable of performing a communication at a desired timing, there is a limit to how much the time required by the DEV to wake-up may be reduced, and, accordingly, it is not possible to adequately lower the power consumption of the DEV. An instance may also arise whereby access timing is set that is unnecessary for the PNC, resulting in a limit to improving a throughput of the communication system.